Enzymatic Formation of a Nonreducing L-Ascorbic Acid a-Glucoside: Purification and Properties of a-Glucosidases Catalyzing Site-Specific Transglucosylation from Rat Small Intestine

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Abstract
We have previously found that some mammalian tissue homogenates can cataluze a unique transglucosylation from maltose to L-ascorbic acid (AA), resulting in a chemically stable AA derivative, L-ascorbic acid α-glucoside (AAG). In the present study, the enzyme responsible for this transglucosylation was isolated from rat intestinal membrane. The formation of AAG was determined by HPLC with an ODS column. The specific activity of AAG-forming enzyme was increaseed in parallel with that of α-glucosidases I and II, were purified to apparent homogeneity. Their enzymological properties showed that they corresponded to maltese (EC 3.2.1.20) and sucrase0isomaltase complex (EC 3.2.148/10), respectively. Both enzymes could form AAG by splitting only maltose among the disaccharides examined, although α-glucosidase I possessed a considerably higher activity thatn the other enzyme. Both AAG formation and maltose hydrolysis were dependent on incubation temperature with the maximal activity at 60°C, but there was an apparent difference between their pH optima. AAG thus formed could also be hydrolyzed by the purified enzymes. From these results, it is concluded that membrane-bound neutral α-glucosidases from rat intentine have site-specific transglucosylase activity to form nonreducing AAG which is distincet from L-ascorbic acid-6-O-α-D-glucoside.